May 17, 2011 -- Men who drink coffee regularly appear to lower their risk of prostate cancer, especially the lethal form, new research suggests.

When the researchers looked at all forms of prostate cancer, ''the highest coffee drinkers had about a 20% lower risk of developing prostate cancer during the follow-up," says researcher Lorelei Mucci, ScD, MPH, associate professor of epidemiology at the Harvard School of Public Health.

The 20% lower risk was found in those drinking six or more cups daily compared to none, she tells WebMD.

Coffee protected even more against the most lethal form of prostate cancer. Among those drinking one to three cups a day, the risk of lethal prostate cancer declined 29%, compared to that of nondrinkers. Among those drinking six or more cups daily, the risk for deadly prostate cancer was reduced 60% compared to that of nondrinkers.

The surprise: the risk reduction held for both regular coffee and decaf, leading the researchers to speculate it's not the caffeine providing the protection.

The study is published online in the Journal of the National Cancer Institute.

About 217,730 new cases of prostate cancer were diagnosed in 2010, according to the American Cancer Society. About 32,000 men died of the disease in 2010.

Checking for Health Benefits of Coffee

Mucci and her colleagues tracked more 47,911 men who participated in the Health Professionals Follow-up Study. The men reported their intake of coffee, both regular and decaf, in 1986, and every four years after that.

From 1986 to 2006, 5,035 men developed prostate cancer. This included 642 with lethal forms of the cancer. Lethal forms were either fatal or were cancers that spread.

The researchers evaluated whether there was a coffee and prostate cancer link. They found it held after accounting for such factors as obesity and smoking.

"We're not exactly sure what the mechanism is," Mucci says. It could be the antioxidants in coffee.

Or it could be substances that reduce inflammation, which has also been linked with higher prostate cancer risk.

It might be compounds that help regulate insulin, she tells WebMD. Higher insulin levels have been linked to higher risk of prostate cancer, she says.

"I think it's probably too early to tell men to start drinking coffee to lower their risks of lethal prostate cancer," she tells WebMD.

However, she adds, it's probably good news for men who already enjoy a cup or more.

More Studies Needed

The study evaluates participants from a long-running, well-regarded study, says Mark Kawachi, MD, director of the Prostate Cancer Center at the City of Hope Comprehensive Cancer Center, Duarte, Calif.

He reviewed the study findings for WebMD. ''Coffee is a very complex beverage, in that there are so many known and unknown chemicals in coffee," he says.

For those advised not to drink caffeinated coffee for other health reasons, he says, the study still holds some good news. "Some of the concerns about drinking so much coffee may be mitigated by the fact they found decaf had the same benefits."

The findings, he tells WebMD, need to be validated in another, independent study.

Likewise, Eric Jacobs, PhD, of the American Cancer Society urged caution in interpreting the findings. "This is a large, well-designed study," he tells WebMD, ''but it is the only study to show a link between coffee consumption and lower risk of fatal prostate cancer." More research is needed to duplicate the finding, he says.

Smoking and being obese are linked with a higher risk of fatal prostate cancer, he says. "So it is fine to enjoy a nice cup of coffee, but avoiding smoking and maintaining a healthy weight are among the surest ways to stay healthy."